Pitting corrosion of mild steel in marine immersion environments

摘要

The conventional wisdom is that for mild and low alloy steels under marine immersion conditions general corrosion typically is the most important form of corrosion. However, this is not always the case and pitting can be quite significant under certain conditions. A review of literature data on pitting of such steels shows that in moderate temperature seawater under 'at-sea' and similar conditions pitting usually is rare in the earlier stages of corrosion but may then increase, substantially. In contrast, in tropical waters substantial pitting occurs within one year of exposure. The data indicate that conventional pit growth models of the type d(t) = A.t~B are inappropriate. Recent exposure trials under 'at-sea' and similar conditions are reported which confirm some of the observations. The recently proposed multi-phase model for marine immersion corrosion is used to re-examine the data. It is shown that under aerobic corrosion conditions pitting for mild steel becomes significant only after anaerobic conditions set in. The commencement of anaerobic conditions is strongly influenced by seawater temperature. Further, to properly account for the various observations, account must be taken of general corrosion in the definition of pit depth.